Summary:
This appears to be caused by the fix of MC-271547. If you control mobs by riding them either by having horses, donkeys, mules, skeleton horses or zombie horses equipped with a saddle or controlling pigs/striders with carrot/warped fungus on a stick, it often glitches when the mob attempts to step up. This was able to occur before 20w20a as well but it only happened rarely when immediately stepping up after falling. This only works if the mob is fast, if it is slow, you are less likely to experience this issue.

Steps to reproduce (horse):

1. /summon horse ~ ~ ~ {Tame:1b,Attributes:[{Name:generic.movement_speed,Base:0.16}],SaddleItem:{id:"minecraft:saddle",Count:1b}}

2. Ride the horse.
3. Attempt to step up 1 block.

Observed results:
It often fails to do so and glitches.

Expected results:
Stepping up should work fine.

Steps to reproduce (pig):

1. /summon pig ~ ~ ~ {Saddle:1b,Attributes:[{Name:generic.movement_speed,Base:1}]}

2. Ride the pig.
3. Attempt to step up with a carrot on a stick.

Observed results:
It often fails to do so and glitches.

Expected results:
Stepping up should work fine.

Video:
horse1.mov